The present invention relates to a nonvolatile memory, for example, a semiconductor nonvolatile memory device such as a flash EEPROM.
A floating gate of the flash EEPROM has to be electrically insulated from a silicon substrate and a control gate. On the other hand, it is necessary for electric charge to travel through a tunnel oxide film below the floating gate at the time of writing/erasure. Because the tunnel oxide film is required to satisfy the conflicting functions, there arises the problem of a limited rewriting frequency because the data rewriting deteriorates the tunnel oxide film, which results in a deteriorated data retention characteristic and the like. Therefore, different efforts have been dedicated to the improvement of the data retention characteristic.
However, in recent years, the tunnel oxide film has been thinner because an operation at a lower voltage is requested. Further, a volume of storage charge determining the data has been decreasing because the semiconductor elements are being scaled down. The thinner tunnel oxide film and the data retention characteristic contradict each other. A suitable technology designed to improve the data retention characteristic is being searched.